1. Technical Field
The present disclosure relates in general to methods of drilling wellbores, for example, but not limited to, wellbores for producing hydrocarbons from subterranean formations, and more particularly to methods of using resistivity data to identify a casing point in a well being drilled using formation resistivity markers obtained from resistivity measurements obtained while drilling one or more subterranean offset wells to identify one or more casing points in the wellbore being drilled while the drill bit advances toward but does not penetrate the formation while drilling the wellbore.
2. Background Art
Formation resistivity measurements are commonly made in oil and gas wells and then used to make decisions about the presence of hydrocarbons, the magnitude of pore pressure, the correlation to formations observed in offset wells, the salinity of formation fluids, porosity of formations, and the presence of permeability. FIG. 1 illustrates graphically the prior art concept of measuring resistivity as a function of depth, showing a typical decrease in resistivity at a depth where increased geopressure (pore pressure) exists (from Eaton, “The Effect of Overburden Stress on Geopressure Prediction From Well Logs”, SPE 3719 (1972)). In shale rocks, resistivity data points diverge from the normal trend toward lower resistivity values, owing to high porosity, overpressured formations.
Existing techniques to measure resistivity are made after the bit penetrates the formation using either electric line logging methods or logging while drilling methods. In either case the formation of interest has already been exposed to the well in order to make the resistivity measurement. This exposure presents problems, including the fact that the condition of the borehole itself and surrounding disturbed formation will have an effect on the very resistivity values being sought, as noted by Hottman et al., “Estimation of Formation Pressures From Log-Derived Shale Properties”, SPE 1110 (1965).
Banning et al. discuss a theoretical application of time-domain electromagnetics (TEM) in a borehole-conveyed logging tool. Banning et al., “Imaging of a subsurface conductivity distribution using a time-domain electromagnetic borehole conveyed logging tool”, Society of Exploration Geophysicists, San Antonio Annual Meeting (2007). See also Published U.S. Patent applications numbers 2005/0092487; 2005/0093546; 2006/003857; 2006/0055411; 2006/0061363; 2006/0061364, and U.S. Pat. No. 6,856,909. Banning et al. state that, theoretically, such a tool may be used to image the conductivity distribution around and ahead of the drill bit at comparatively large distances from the borehole. However, Banning et al. do not disclose or suggest use of resistivity measurements in front of a drilling bit to identify a casing point in a wellbore being drilled, while the drill bit advances in the wellbore being drilled toward but does not penetrate a region of interest in the formation, using formation resistivity markers obtained from resistivity measurements obtained while drilling one or more subterranean offset wells. Banning et al. also do not disclose or suggest using resistivity in front of the bit measurements in the wellbore being drilled, in addition to resistivity measurements obtained from the one or more offset wells, to identify casing points in the wellbore.
It is known in wellbore planning and drilling operations to study data from offset wells to develop and validate geomechanical stress models, and adjust casing points and mud weights to meet well challenges. See for example Brehm et al., “Pre-drill Planning Saves Money”, E & P, May 2005. An offset well is an existing wellbore close to a proposed well that provides information for planning the proposed well. In planning development wells, there are usually numerous offsets, so a great deal is known about the subsurface geology and pressure regimes. A casing point is a location, or depth, at which drilling an interval of a particular diameter hole ceases, so that casing of a given size can be run and cemented. Establishing correct casing points is important in the design of the drilling fluid program. Conventionally, a casing point may be a predetermined depth, or it may be determined by a pressure hunt, selected onsite according to geological observations or dictated by problems in the open hole section. In many cases, weak or underpressure zones must be protected by casing to enable mud-weight adjustments that control unstable formations or overpressure zones deeper in the wellbore. A pressure hunt is used to evaluate various well parameters to identify when the pore pressure in a drilling well is changing. The purpose is to detect the pore pressure transition (usually from lower to higher pressure) and safely set casing in the transition zone to maximize wellbore strength.
To avoid or reduce undesirable consequences, it would be advantageous if resistivity measurements from one or more offset wells could be used to determine a casing point or points in a wellbore being drilled before the bit exposes the formation during drilling the wellbore. The methods and apparatus of the present disclosure are directed to these needs.